The present invention generally relates to a non-impact recording method and apparatus, and more particularly to an electrothermic recording method and apparatus, wherein an electroconductive thermal-transferable ink material is applied to a receiving surface in areas where the ink material is softened in an image pattern by heat generated within the ink material.
Conventionally, several electrothermic printing methods and apparatuses are known in which a ribbon containing or coated with a pigmented and thermal-transferable material is superimposed on plain paper, and the thermal-transferable material is locally softened in image form in response to image-delineating electric currents applied thereto, and is then transferred to the plain paper as dots or lines.
More specifically, U.S. Pat. No. 2,713,822 discloses a recording method of the above-mentioned type which employs a transfer sheet comprising a base sheet of electroconductive material having on one surface a coating of a relatively electrically non-coductive image-forming fusible material, and having on the opposite surface a resistive layer which has substantial electrical resistance as compared with the base sheet. In that method, a voltage is applied between a point on the resistive layer and an edge of the base paper, by means of an electrode, which voltage causes a current to flow between the point and the base paper edge through the connecting portion of the base sheet, the length of which portion varies in accordance with the location of the point. The Joule's heat generated in the portion of the resistive layer immediately below the electrode causes the image-forming fusible material to melt and the melted material is transferred to the underlying planographic printing plate. This method has the disadvantage that the resistance between the point where the electrode is in contact with the resistive layer and the base sheet edge changes as the position of the electrode changes, and accordingly the amount of the Joule's heat generated changes, depending upon the position of the electrode. The result is that inconsistent printing quality is caused since in some portions excess transfer of the image-forming fusible material takes place, while in other portions, the transfer is insufficient, due to variations in the extent of ink melting.
As an improvement on the above method, U.S. Pat. No. 3,744,611, discloses an electrothermic printing device including a printing head having at least two electrodes of different electrical potentials, which are spaced a predetermined distance from each other and are in contact with a ribbon whose thermal-transferable ink layer can be printed on a receiving surface in areas where the ink is softened by the Joule's heat generated by the current flowing through the electrodes. Specifically, in this reference, two types of printing heads are disclosed for use in this printing device. The first printing head comprises a first electrode means comprising an electrode member which has an elongated opening, energizable to a first electrical potential, and serves as a return electrode, and a second electrode means comprising a plurality of wire probes each selectively energizable to a second electrical potential, which probes serve as the recording electrodes and are positioned in the aforementioned elongated opening spaced apart from one another. The second printing head comprises a row of selectively energizable points which serve as the recording electrodes, and two elongated electrodes which serve as the return electrodes and are disposed parallel to the row and positioned on the opposite sides of the row. In these printing heads, the recording electrodes are essentially surrounded with a single return electrode or a pair of return electrodes by either projecting the recording electrodes through an opening in the single, massive return electrode, or by fixing two parallel, elongated return electrodes around a row of recording electrodes, one on each side.
In the above U.S. patent, however, there is no mention of various factors having an effect on apparatus design, printing quality and energy consumption, including the relationship between the Joule's heat generated at the recording electrodes and that at the return electrode, the effect of distance between the recording electrodes and the return electrode, and the relationship between the contact areas with the ink layer of the recording electrodes and the contact area with the ink layer of the return electrode.
In U.S. Pat. No. 3,719,261, there is disclosed a printing method using electroconductive fusible ink. In this method, an electrically anisotropic ink support material--i.e., one in which electric conductivity varies with the direction through the material--is used. In this case, the electric conductivity is greater in the transverse direction (normal to the surface) than in the superficial direction (parallel with the surface). One surface thereof is covered with a solid and fusible electroconductive ink. Pairs of points defining the desired outline are selected on the support. One point of each selected pair is connected to one pole of a current source and the other point of each selected pair is connected to the opposite pole of the source, thus causing current to flow between the points of each selected pair. The ink melts along the current path and the molten ink is picked up by the paper, previously placed in contact with the support, thereby printing the outline defined by the selected pair of points.
In this method, since the melting of the ink is not limited to a point, but takes place along the entire current path, causing the entire molten-ink line to be transferred to the paper, there is a limitation on increasing the obtainable image resolution.